In transmitting voice-frequency samples and other message signals by the PCM technique from a variety of sources to a corresponding number of destinations, it is customary to interleave the bits of binary words from different sources, representing the quantized instantaneous signal amplitudes, in a recurrent frame for transmission at a higher bit frequency or cadence over a common signal path. Several such signal paths may, in turn, be merged with interleaving of the bits of their frames in a secondary frame; again, the bits of a plurality of such secondary frames may be interleaved in a tertiary frame, and so on.
Thus, a transmitting terminal at the input end of a PCM link, designed to carry a bit stream with a recurrent higher-order frame to a receiving terminal at its output end, may serve a plurality of incoming lines carrying contributory bit streams each in the form of a recurrent lower-order frame. All these incoming bit streams will generally have the same cadence, e.g. of 8.448 Mbits/sec in the case of incoming secondary frames, so that the transmission rate of the interleaved message bits from four incoming lines traveling over the common PCM link in a higher-order (e.g. tertiary) outgoing frame ought to be 33.792 Mbits/sec. In practice, however, the higher-order frame must include additional time slots designed to accommodate ancillary bits for supervisory signals which accompany the message signals from the lower-order frames to insure synchronization, provide distribution information (i.e. to identify the contributing bit streams in the composite frame) and convey alarm indication in the event of a malfunction. It is therefore necessary to adopt a higher bit cadence, e.g. of 34.368 Mbits/sec, for the composite bit stream passing from a multiplexer at the transmitting terminal to a demultiplexer at the receiving terminal.
If the departing bits are delivered by the multiplexer at a frequency F.sub.m and if each composite frame synthesized from the bits of four frames contains X message bits and x ancillary bits for each contributing frame, then the mean recurrence rate F.sub.d of the message bits of each contributing frame in the composite frame will be given by EQU F.sub.d = F.sub.m .multidot. (X-x/4X)
that mean frequency F.sub.d, however, generally does not correspond to a whole number of bits or time slots per frame period wherefore it is customary to resort to a technique known as "bit stuffing", i.e. the use of a blank time slot devoid of message information during certain frames. This produces two different values for the mean frequency F.sub.d which, when averaged over a multiplicity of frame periods, correspond to the bit cadence f.sub.m in each contributing frame.